Embodiments of the invention generally relate to regenerative thermodynamic cycles, e.g., regenerative Brayton cycles, and more particularly to power generation systems e.g., gas turbine power plants, which use the regenerative Brayton cycles, and methods for operating the systems.
Regenerative thermodynamic cycles are typically implemented in gas turbines and micro-turbines to improve the cycle (e.g., Brayton cycle) efficiency beyond what is otherwise achievable with a simple cycle machine. In current regenerative gas turbine cycles, a partial replacement of the fuel energy is achieved by regeneratively transferring energy from the exhaust gases via heat exchangers to a fluid discharging from the compressor. The compression ratio in such a machine is low enough that the temperature of the exhaust gases leaving the turbine and entering the regenerator is higher than the compressor discharge air to be heated therein. A substantial improvement in the efficiency of the gas turbine cycle has been realized.
Further improvements to these gas turbine cycles have been achieved by using various processes and configurations, for example, multistage compression with intercooling, multistage expansion with reheating, and recompression. However, even in such recuperated and recompression cycles, the thermal efficiency is limited by the fact that the turbine exhaust gas temperature can never be cooled below that of the compressor discharge air, or else the heat will flow in a reverse direction (to the exhaust gases), decreasing the efficiency of the system.
More recently, there has been an increased interest in the use of supercritical working fluids, such as supercritical carbon dioxide, in closed thermodynamic power generation cycles. For example, a supercritical Brayton cycle power generation system offers a promising approach for achieving a higher efficiency and more cost-effective power conversion when compared to the existing steam-driven power plants and gas turbine power plants. A supercritical Brayton cycle using supercritical carbon dioxide as the working fluid, has a lower compression work value than that of a conventional gas Brayton cycle due to the low compressibility of the fluid near the critical point. Furthermore, a supercritical Brayton cycle using supercritical carbon dioxide as the working fluid can be regeneratively heated to a higher temperature than that of a steam Rankine cycle, enabling a higher efficiency.
In order to achieve a high thermodynamic efficiency in a regenerative power cycle, it is desirable to regeneratively heat the working fluid to a temperature higher than its critical temperature. This is challenging because the specific heat of the fluid near its critical temperature is higher than the specific heat of a regenerative fluid. One solution to this challenge is a recompression cycle. In a recompression cycle, a recompressor (i.e., an additional compressor) is generally used to compress a fraction of an exhaust fluid before the heat is removed by a precooler (i.e., when the temperature of the fluid is near the exhaust temperature), and mix it with a cooled and compressed fluid after leaving from a low temperature recuperator. Although the two parallel compressors have quite different inlet conditions, one must operate at nearly the same pressure ratio to avoid impacting the performance of the other compressor. A relatively small difference in the pressure ratio can surge one of the compressors and shutdown the system. Furthermore, while maintaining adequate surge margin, the recompression cycle adds challenges for balancing the performance of the two compressors. In short, the recompression cycle has the potential for improving the efficiency of the system, however the cycle is mechanically complex and difficult to control because of the two separate compressors operating at different conditions.
Therefore, alternative configurations for the regenerative thermodynamic cycles are desirable, which provide advantages over conventional thermodynamic power generation cycles, typically, used in the power generation systems e.g., gas turbine power plants. In particular, alternative regenerative thermodynamic cycles are desirable, which maintain high efficiency without a recompressor.